Parahydrogen-Induced Polarization in Gas-Phase Heterogeneous Hydrogenation of Epoxides

Hydrogenation of epoxides is a reaction of both fundamental and practical importance. Herein, gas-phase heterogeneous hydrogenation of saturated and unsaturated epoxides over titania-supported Pt, Pd, and Rh catalysts was investigated using the parahydrogen-induced polarization (PHIP) technique. Hydrogenation of saturated epoxides (ethylene oxide and propylene oxide) yielded alcohols, carbonyl compounds, alkenes, and alkanes. While all three catalysts under study isomerized epoxides to carbonyl compounds in the presence of H₂, only Pd/TiO₂ showed catalytic activity under inert atmosphere, implying that on Pt and Rh catalysts, the epoxide C–O bond can be cleaved only simultaneously with H atom addition to form alkoxy species. PHIP effects were detected for propane and propylene but not for alcohols. Hydrogenation of bifunctional epoxide 3,4-epoxy-1-butene yielded 11 reaction products. Adsorption of the reactant via its CC bond is followed by hydrogen addition, resulting in the formation of 1,2-epoxybutane, which does not react further. Alternatively, the reactant can be adsorbed via both its CC bond and the oxygen atom, leading to the formation of crotonaldehyde, butanal, crotyl alcohol, 1-butanol, methyl 1-propenyl ether, methyl propyl ether, 1,3-butadiene, 1-butene, 2-butene, and butane. Based on the PHIP effects observation, the mechanisms of the corresponding processes were proposed.